1. Field of the Invention
The present invention relates to electric double layer capacitors having large capacitance and high power, and in particular, relates to a technique to prevent gas generation due to decomposition of the electrolyte solution used in the capacitor.
2. Background Art
An electric double layer capacitor has characteristics such as long service life, high cycling characteristics, characteristics of charge and discharge with large current, and operates under a wide range of temperatures since there are no chemical reactions during charge and discharge of the capacitor as there are in a conventional secondary battery. Therefore, this type of capacitor is attracting much attention as a new type of storage battery or as a driving power supply for automobiles and devices. In particular, electric double layer capacitors having large capacity and high power are being developed.
An example of such an electric double layer capacitor is shown in FIG. 1. As shown in FIG. 1, the capacitor 1 includes a cylindrical case with a bottom 2, polarized electrodes 5 contained in the case 2, a separator 6 disposed between the electrodes, and electrolyte solution filled in the case 2. The cylindrical case with a bottom 2 includes an aluminum body having an opening part 2a and an aluminum cover plate 4 which closes the opening part 2a. A part between the outer circumference of the cover plate 4 and inner circumference of the body is sealed with a sealing material. The polarizing electrodes 5 include a mixture of an activated carbon for an electrode, a conductive filler, and a binder.
Conventionally, as the electrolyte solution used as a material of the electric double layer capacitor, an aqueous electrolyte solution and a nonaqueous electrolyte solution can be mentioned. In particular, since high voltage use is required to improve energy density of the electric double layer capacitor, a nonaqueous electrolyte solution which can be charged and discharged at relatively high voltage has been widely used. As the nonaqueous electrolyte solution, a solution in which various conditions such as low-temperature properties, solubility of salts, dielectric constant, safety, electrolyte solution decomposition properties, boiling point, cost and the like are improved is required. As a material which meets these requirements, an electrolyte solution in which propylene carbonate is mainly used as a solvent and a quaternary ammonium salt is added as a supporting salt, may be mentioned.
However, in an electric double layer capacitor having an electrolyte solution containing propylene carbonate and an alkali activated carbon, in the case in which it is charged and discharged repeatedly under high voltage, there is a problem in that the electrolyte solution is gradually decomposed by an electric current under high temperature conditions and carbonic acid gas is generated.
To solve such problems, methods in which various types of materials are added to suppress the decomposition of the electrolyte solution have been suggested. Practically, a nonaqueous electrolyte solution containing γ-butyrolactone or γ-valerolactone in which electrolysis is reduced (see Japanese Unexamined Patent Application Publication No. 2001-217150), a nonaqueous electrolyte solution containing fluorobenzene (see Japanese Unexamined Patent Application Publication No. 2004-6803), a nonaqueous electrolyte solution containing diphenyls (see Japanese Unexamined Patent Application Publication No. 2004-146610) and the like are disclosed.
However, in these solvents, the effects of suppressing gas generation are insufficient. In addition, by depending on additives, the initial capacitance of the capacitor is reduced or internal resistance is increased. Furthermore, the capacitance decreases over time due to the adsorbing of additives or reaction products to additives in the activated carbon. Additionally, dehydration and purification of the additives are required to add the additives to the electrolyte solution, and the cost is increased by the solution containing multiple components.